The Crystal Empire was definitely ruled by Kings/Queens in the past (it was a Queen who started the Crystal Fair tradition, and as for an example of a Crystal King...see Sombra ); as I see it, Cadance is not yet the Queen, but only because there hasn't been time for a formal coronation yet.

In most European monarchies, when a ruling Queen ascends to the throne, her husband usually gets the title of "Prince" or "Duke" or some other important noble title. As Twilight is linked to the royal families of the Crystal Empire and of Equestria by her brother's marriage, and not by blood relation, it's unlikely for her to get any official "princess" title for now.